A circuit component that is particularly susceptible to damage from an electrostatic discharge (ESD) event is an N-channel metal oxide semiconductor (MOS) transistor connected to a signal input/output pad. In a "two-terminal ESD event," a current surge can be injected into a pad connected to one source/drain terminal of the output driver MOS transistor and is extracted from a second pad connected to the other source/drain terminal, so that almost the full ESD voltage develops across the terminals.
In FIG. 1, ESD protection from a surge that enters at a signal input/output pad 10 is provided by a low-impedance shunt 12. The shunt provides a path from the signal pad to a ground bus or ground plane 14 connected to a second pad 16. The low-impedance shunt reduces the risk of damage to a driver transistor 18 and a receiver transistor 20. A diode 22 is incorporated into the circuit to conduct ESD current injected at the pad 16 for exit at the signal pad 10.
In N-well CMOS circuitry, the ground bus 14 is typically connected to the semiconductor substrate of the integrated circuit and to sources of N-channel transistors of core circuitry 24. For each additional signal input/output pad 26, a separate low-impedance shunt 28 and a separate diode 30 are employed to protect the driver transistor 32 and receiver transistor 34 associated with the signal pad. Also shown in FIG. 1 is a positive voltage supply connected at a supply pad 36 to provide V.sub.DD for operation of the circuitry.
One difficulty with the circuitry of FIG. 1 is that switching noise generated by operation of transistors 18, 20, 32 and 34 may affect the core circuitry 24 because of the common connection to the ground bus 14. A known solution to isolating switching noise is to provide a separate ground bus for the switching transistors. In FIG. 2, a switching ground bus (or switching ground plane) 38 is connected to pad 40. On the other hand, core circuitry 42 is connected to a substrate ground bus 44 that is joined to a separate pad 46. The source 48 of an N-channel MOS transistor 50 is connected to the switching ground bus 38, while the drain 52 is connected to a signal input/output pad 54. The switching ground bus is sometimes referred to as "dirty ground" or "isolated I/O ground." The substrate ground is also referred to as "clean ground" or "core ground." The two ground buses 38 and 44 are held at the same potential by connections within or outside of the integrated circuit package that houses the circuitry of FIG. 2. A diode is typically included to conduct current from the substrate ground bus 44 to the switching ground bus 38. The diode is typically a parasitic diode formed by the N-channel driver pull down sources in a P-type substrate.
Moving the source 48 of the driver transistor 50 from the "clean" substrate ground bus 44 to the "dirty" switching ground bus 38 isolates the switching noise, but it also reduces the ESD protection provided by circuits having a low-impedance shunt 12 of the type shown in FIGS. 1 and 2. For two reasons, the conduction of ESD current from a signal pad 54 to an isolated ground bus 38 to which the sources 48 of driver transistors are connected is more severe than the conduction of ESD current from a signal pad to a substrate ground to which the sources are connected. Firstly, in the isolated ground approach the drain-to-source voltage experienced by the driver transistor 50 increases to the sum of the signal-to-ground voltage plus the voltage drop across the additional forward-biased parasitic diode from clean ground to dirty ground. Secondly, in the clean-ground approach, the source and drain junctions of the output drivers are both at zero or reverse bias until one of the junctions reaches the breakdown voltage and the driver "snaps back," whereas the substrate-to-source junction in the dirty-ground approach of FIG. 2 is immediately driven into forward bias by the current that passes from the clean substrate ground bus 44 to the dirty switching ground bus 38.
What is needed is an ESD protection circuit for integrated circuits that use isolated switching buses, wherein ESD current is non-destructively conducted whether injected at an input/output pad or a common bus.